The present invention relates to ionic liquids useful as electrolytes in connection with energy storage devices such as supercapacitors. More particularly, it relates to organosilicon phosphorus electrolytes.
Supercapacitors, sometimes also referred to in the literature as “electric double-layer capacitors”, “electrochemical capacitors” or “ultracapacitors” are well known. Supercapacitors provide energy storage as well as pulse power delivery. This is useful in many applications, such as in connection with automotive starters and hybrid automotive vehicles.
One well known type of supercapacitor is depicted in FIG. 1. This drawing shows a supercapacitor 10 having two electrodes 11 which are kept from electrical contact with each other by a separator 12. There are current collectors 13 at opposite ends of the device. The electrodes consist of a porous material 14 and an electrolyte 15. Both the separator 12 and the porous material 14 are typically immersed in the electrolyte 15.
Current collecting plates 13 are in contact with the electrodes 11. Electrostatic energy is stored in polarized liquid layers, which form when a potential is applied across two of the electrodes. A “double layer” of positive and negative charges is formed at the electrode-electrolyte interface. The electrolyte allows ions to move freely through the separator.
To be optimally effective for certain applications, supercapacitors must, among other properties, have low internal resistance, store large amounts of charge, be physically strong, be stable at desired (preferably high) voltages, and be otherwise compatible with the usage environment. Therefore, there are many design parameters that must be considered in construction of such devices.
Aqueous and some organic electrolyte solutions have been proposed for use in supercapacitors. Aqueous electrolytes provide relatively low series resistance, improving the time constant of a supercapacitor and providing high power densities. However, they are often not stable at the operating voltages exceeding the electrolysis voltage of water (1.23 V).
Organic liquid electrolytes used in supercapacitors should preferably have higher ionic conductivity. As an example, acetonitrile provided high ionic conductivity. However, acetonitrile is a hazardous flammable and toxic material, which produces highly toxic products (HCN and CO) upon combustion and thermal decomposition.
Some other previously used organic liquid electrolytes have been based on alkyl carbonates (ethylene carbonate, propylene carbonate, and γ-butyro-lactone, or dimethylcarbonate, diethylcarbonate, and ethylmethylcarbonate, for example) which are highly flammable. Some also have lower ionic conductivity as compared to aqueous electrolytes or electrolytes based on acetonitrile, and this causes higher internal losses of stored energy and power density of the supercapacitor.
In A. Balducci, The Use of Ionic Liquids As Solvent-free Green Electrolytes For Hybrid Supercapacitors, 82 Applied Physics 627-632 (2006), there was a discussion of using (CF3SO2)2N− (“TFSI”) as an anion portion of salts containing cyclic cationic nitrogen moieties, as electrolytes for supercapacitors.
Our laboratory also recently reported, in U.S. patent application publication 2007/0076349, that polysiloxanes could have utility as electrolytes for supercapacitors and other energy storage devices.
In Z. Li et al., A New Room Temperature Ionic Liquid 1-butyl-3-trimethylsilylimidazolium Hexafluorophosphate As A Solvent For Extraction And Preconcentration Of Mercury With Determination By Cold Vapor Atomic Absorption Spectrometry, 71 Talanta 68-72 (2007) there was a discussion of using PF6− as an anion with a cationic organosilicon compound having a cyclic nitrogen containing moiety, as a solvent for extraction.
In U.S. Ser. No. 11/865,089, filed Oct. 1, 2007, we recently reported on the use of organosilicon amine-based electrolytes, primarily for use in energy storage devices such as supercapacitors. These have certain advantages. However, still other improvements are desired with respect to electrolytes having varied properties.
In E. Frackowiak et al., Room-temperature Phosphonium Ionic Liquids For Supercapacitor Application, 164104-1-164104-3 (86 Applied Physics Letters) (2005) there was a discussion of using phosphonium salts for supercapacitor electrolytes, where the anion for such salts was (CF3SO2)2N−.
Notwithstanding these developments in the art, there is a need for additional improvements with respect to electrolytes for supercapacitors and batteries, particularly with respect to providing electrolytes which do not have high flammability and have high thermal stability.